1. Field of the Invention
The present invention relates to an electrostatic chuck for heating a semiconductor wafer as an object to be heated, and used in an apparatus such as a CVD apparatus and a sputtering apparatus in a manufacturing process of semiconductor devices, or an etching apparatus for etching a formed thin film.
2. Description of the Background Art
There exist various well-known technologies in the manufacture of devices for semiconductors, for instance technologies that involve forming polysilicon films and/or oxide films, conductive films, dielectric films and the like on a semiconductor wafer, using a CVD apparatus and/or a sputtering apparatus, or, conversely, technologies involving etching of the foregoing thin films using an etching apparatus. In order to ensure film formation and/or film etching quality in the above apparatuses it is necessary to keep the semiconductor wafer, as the objective work for heating, at a desired constant temperature, and hence a heater for heating the semiconductor wafer is required to perform such temperature control.
In semiconductor wafer heating are used electrostatic adsorption apparatuses for fixing the semiconductor wafer onto the heater, in a reduced-pressure atmosphere. The material of the electrostatic chucking apparatus shifts from resins to ceramics as the temperature of the process increases (Japanese Patent Application Laid-open No. S52-67353).
Wafer heaters having electrostatic chucking functionality have also been proposed recently. These wafer heaters combine a monolithic-type ceramic wafer heater with an electrostatic chucking apparatus. For instance, an electrostatic chuck using TiO2-containing alumina in an insulating layer of an electrostatic chucking apparatus is employed in low-temperature stages, as during etching (Toshiya WATANABE “Electrostatic Force Characteristics of Ceramic Electrostatic Chucks”, NEW CERAMICS (1994) No. 2, p. 49-53, 1994), while an electrostatic chuck using pyrolytic boron nitride in an insulating layer of an electrostatic chucking apparatus is employed in high-temperature stages, as during CVD (Japanese Patent Application Laid-open Nos. H04-358074, H05-109876, H05-129210, and H07-10665).
As the ceramic substrate is usually used a sintered body obtained through sintering of a starting-material powder to which a sintering auxiliary agent is added. Such a ceramic substrate undergoes strain as a result of thermal stress caused by the differences in the coefficient of thermal expansion among different materials such as a heat-generating material and the like. When, for instance, a semiconductor wafer or the like is placed upon, and heated by, a substrate of an electrostatic chuck with a built-in electric heating means comprising different materials such as a ceramic substrate, a heat-generating material or the like, the above-described strain can lead to deficient surface matching with the wafer, thereby disrupting temperature distribution.
Known methods for suppressing such strain include methods in which the rigidity of the ceramic substrate is enhanced by increasing the thickness thereof, and in which affixing of the substrate to a mount is reinforced. The attempt to suppress strain that way, however, is problematic in that thermal stress accumulates inside the ceramic substrate and in the interfacial boundary between the ceramic substrate and the heat-generating material, so that, upon repeated rising and falling temperature cycles, ruptures occur in the boundary between sintered particles and/or in the boundary between the ceramic substrate and the heat-generating material. A thicker ceramic substrate means a greater heat capacity, which is problematic in that more time is required as a result for raising and lowering the temperature.
In order to solve these problems, integration-type resistance-heating multilayer electrostatic chucks with built-in electric heating means have been developed in which an insulating layer comprising pyrolytic boron nitride is formed by thermochemical vapor-phase deposition (thermal CVD) on a base plate comprising carbon or a carbon-based composite material, onto the insulating layer being further bonded a heater pattern comprising a pyrolytic graphite film formed by thermal CVD, the heater pattern being covered by a compact layer-like protective film of pyrolytic boron nitride or the like (Japanese Patent Application Laid-open No. H09-40481).
Such integrated-type resistance-heating multilayer electrostatic chucks with built-in electric heating means are highly pure and chemically stable, and are also strong towards thermal shock, for which reason they are used in a wide range of fields where abrupt temperature changes are required. More specifically, such chucks are widely used in the field of semiconductor wafer manufacture, for instance, where the semiconductor wafers or the like are subjected, wafer by wafer, to a continuous process that involves gradual temperature changes. Such multilayer electrostatic chucks with built-in electric heating means are widely used because they are manufactured by CVD throughout, as described above, and have hence no grain boundaries, are free of degassing, and exert no negative influence during processes that involve heating in vacuum.
These multilayer electrostatic chucks with a built-in electric heating means have a layer structure of various materials, and comprise for instance a support substrate, an electrode layer, wiring, a dielectric layer, and an electroconductive layer. Warping is likely to occur in any stage owing to processing strain. When a wafer or the like in this warped state is fastened to another surface and is heated there occur problems such as impaired matching between surfaces and uneven temperature distribution.
Conventional technologies have resorted to flattening the workpiece-chucking surface through mechanical processing, to achieve a uniform attracting force in order to eliminate such warping. However, just flattening the workpiece-chucking surface alone leads eventually to a less flat workpiece-chucking surface, and hence in a lower attracting force, owing to the resulting in-plane film thickness distribution and the influence of warping of the opposite side when the electrostatic chuck is fixed on an apparatus to be mounted thereon.
In the case of dielectric layers having high volume resistivity values, in particular, such attracting force is dominated by Coulomb forces, and increases in proportion to the square root of the thickness of the dielectric layer. That is, the thickness distribution of the workpiece-chucking surface is an extremely important factor bearing directly on the in-plane attracting force distribution, with a larger film thickness distribution resulting in a greater variation of in-plane attracting force, which ultimately gives rise to temperature distribution unevenness in the heated body.
When attempting to achieve a uniform film thickness distribution in order to increase the attracting force, the workpiece-chucking surface adopts a concave or a convex shape, which in turn gives also rise to temperature distribution unevenness in the heater body. When the workpiece-chucking surface is concave/convex, moreover, attracting speed and attracting strength vary in accordance with the concave/convex contour, being different in the central portion and the periphery of the heater body adsorbed to the workpiece-chucking surface. This gives rise to frictional forces between the heater body and the workpiece-chucking surface, from the center towards the periphery, and results in increased particle formation. Suppressing particle formation as much as possible is an extremely important issue when it comes to increasing semiconductor manufacturing yields.